The qualifying income required to purchase a median-priced home in Boston, New York, Los Angeles and San Diego now exceeds $105,000, though the median income in those cities ranges from $46,800 to $55,200, according to a study by the Center for Housing Policy and John Burns Consulting in Irvine, Calif. The Center for Housing Policy is a research affiliate of the National Housing Conference.

This coastal imbalance in home prices and incomes is not evident in such inland cities as Kansas City, Mo.; Dallas, Texas; and Detroit, Mich., where median-income households can still qualify to buy a median-priced home, according to the data.

"One thing is clear: Home prices in many U.S. markets are so far out in front of middle-class incomes that only the upper echelon of would-be home buyers can afford the median-priced houses in their communities," McGee said in an announcement today.

And the run-up of home prices over the past eight years is unprecedented in the history of the nation's housing market, creating uncertainty as to what lies ahead. "Nobody seems to know what will happen next," she said. "The real question in the minds of real estate professionals and homeowners alike is not whether bubbles exist, but what form the coming correction will take. Will we see a soft landing, or are we headed over the edge of a cliff?"

The price-appreciation in many markets, said McGee, "has finally left the atmosphere, and is posed for reentry."

McGee cited data from John Burns Consulting showing that a majority of income is going toward housing in some markets. In San Diego, for example, an estimated 54 percent of income is going to service home loans, while housing costs claim 52 percent of income in Orange County, Calif., 41.9 percent of income in Boston, and 42.5 percent of income in New York.

"These are dangerous ratios," McGee said, "and these figures are fueling the talk of bubbles in these housing markets."

But unlike the real estate collapse of 1989, when the inventory of homes for sale ranged from nine months to a year in some markets, there is a comparatively low inventory of homes on the market today, McGee noted. That could be changing. In Orange County, new listings of homes in the "$550,000 to $750,000 range jumped from 248 in March to 964 in July, according to the local MLS. And Realtors in Long Island, N.Y., have reported a 30 percent increase in time on market, Foreclosures.com also reported.

McGee said she expects a continued rise in foreclosures activity as the market turns.